Water utilization and containing systems such as boilers, cooling towers and heat exchangers are notoriously susceptible to scale build-up and formation due to the presence of impurities in the water. Such impurities are normally the water hardening compounds, such as the various calcium salts and iron oxide found in most water supplies, domestic and commercial. Scale build-up on heat transfer surfaces in the aforementioned equipment causes much expense to the owner as well as possible exposure to dangerous conditions for those persons located near such equipment. Scale build-up decreases the rate of heat transfer through the surfaces, and hence contributes to an overall loss of efficiency as well as increased energy costs. Scaled heat transfer surfaces are susceptible to corrosion beneath the scaled area since corrosion control agents are unable to effectively contact the metal surface. Furthermore, scale acts as an insulator, resulting in inefficient heat transfer. This condition leads to both increased fuel costs and decreased system efficiency. System component failures can occur due to the higher than normal temperatures at the heat transfer surfaces. Repair costs of component failures include labor, replacement parts cost, and plant down-time. Experience has shown that the use of sophisticated water pre-treatment techniques or careful chemical treatment do not always preclude the possibility of scale formation.
The primary components of most water impurity caused scales in the equipment mentioned above are calcium salts (e.g. calcium carbonate, calcium orthophosphate, calcium sulfate and calcium bicarbonate) and iron oxides. These compounds are present in most domestic and commercial water supplies, having been found both in surface and subterranean water sources.
The present state of the art scale prevention includes the use of chelants, polymeric conditioners and gluconates as sequestering agents. Chelants such as tetrasodium ethylene diamine tetra acetic acid (EDTA) and trisodium nitrilotriacetate (NTA) stoichiometrically sequester calcium and iron ions. Stoichiometric treatments require significant concentrations to prevent scale formation, particularly when the water source contains substantial quantities of calcium and iron ions. Gluconates and polyelectrolytes are normally threshold treatments. Threshold treatments are generally effective in controlling large numbers of ions per unit of treatment, and the economy of using such threshold treatments is readily apparent.
Scale removal is normally accomplished by descaling with acid treatments. Organic acids such as citric and sulfamic or mineral acids such as hydrochloric are frequently used to remove scale of the type mentioned above. However, in acid descaling, the system must be shut down, drained, acid cleaned, rinsed, drained and retreated. Acid descaling thus requires either waiting for a convenient time to shut down operations or possibly interrupting production schedules on an emergency basis to achieve cleaning. Either alternative is known to be costly. Since draining of the system is often required, an added cost is the replenishing of chemical treatment of the water, once the system is clean.